1. Field of the Invention
The present invention relates to a structure of a downlink control channel and method for allocating a downlink control channel in a broadband wireless access system and, more particularly, to a method and apparatus for allocating a downlink control channel in a system using a fractional frequency reuse (FFR).
2. Description of the Related Art
Currently, the explosive increase in demand for radiowave resources, as well as the advancement of a communication environment, accelerates the exhaustion of available frequency resources in a frequency band lower than 3 GHz.
In an effort to overcome the exhaustion of frequency resources, a frequency reuse technology has been proposed.
In a communication system, a frequency reuse refers to reusing the same frequency at a certain distance from a neighboring cell or sector while minimizing interference.
FIG. 1 illustrates a cell frequency disposition when a frequency reuse factor K is 3.
As illustrated, in the overall system using the three frequencies G1, G2, and G3, cells using the same frequency are disposed to be as far as possible, in order to minimize interference between neighboring cells that reuse the same frequency
The frequency reuse factor K is a value indicating how many cells or sectors the same frequency is reused in, and as the frequency reuse rate increases, the distance between cells or sectors using the same frequency is longer, reducing interference caused by using the same frequency, which, however, accompanies degradation of efficiency in terms of resource utilization. Thus, in order to enhance efficiency of frequency resource utilization, it is preferred to use such that the frequency reuse rate is close to 1, but when the frequency reuse rate is 1, a problem arises in that interference between neighboring cells or sectors occurs in the boundary of a cell or sector.
In order to improve the problem, a fractional frequency reuse (FFR) has been introduced.
The FFR uses that, in general, a terminal located at a central area of a cell and a terminal located at a boundary area differently affected by neighboring cells. Namely, the terminal located at the central area is not distant from a base station, degradation of receive sensitivity of a signal component due to a path attenuation is low, but relatively distant from a neighboring interference base station, greatly affected by the path attenuation, resulting in a decrease in the influence of the same channel interference. Meanwhile, in case of the terminal located at the boundary area of the cell, because its serving base station and interference base station are located at a similar distance, a signal component and an interference component are received with a similar sensitivity, increasing the influence by the same channel interference. Thus, in the FFR scheme, the terminal located at the central area of the cell is allowed to have a frequency reuse rate of 1, while the terminal located at the boundary area of the cell is allowed to have a frequency reuse rate of more than 1, to guarantee a receive performance of the user terminal located at the boundary area to a degree.
FIG. 2 illustrates cell frequency disposition employing the FFR scheme.
As shown in FIG. 2, the overall frequency resources are divided into a portion FP0 where the frequency reuse rate is 1 and portions FP1, FP2 and FP3 where the frequency reuse rate is 3. FP0 is commonly used at the central area of each cell, and FP1, FP2 and FP3 are used only in one of three neighboring base stations.
In the FFR system, the overall bands may be divided into a band (common band) where the frequency reuse rate is 1 and a band where the frequency reuse rate is 3.
Meanwhile, when medium access protocol (MAP) information including control information is transmitted to a user, one MAP area exists in each subframe. In the system supporting the FFR including the frequency reuse rate of 1 and the frequency reuse rate of 3, the MAP area may be positioned at all the four frequency partitions or at a single frequency partition.
When the MAP area is allocated to all the frequency partitions, MAP information elements (IEs) positioned at the first partition include resource allocation information regarding the partition 1, and MAP IEs positioned at the second partition include resource allocation information regarding the partition 2. In this case, overhead of control channels with respect to data can be advantageously distributed to the respective channels, but because common non-user specific control information (NUSCI) must be included in every partition, overhead is generated, and particular areas (i.e., areas which are not power-boosted in a Reuse 3 area) has a weak power strength, possibly generating a decoding error in a terminal located at a cell edge. Also, a problem arises in that processing overhead of the terminal that decodes the non-user specific MAP with respect to every partition increases.
In addition, when the MAP area is allocated only to one partition among the plurality of frequency partitions, repetition of the common control information can be advantageously reduced, but causing a problem in that because the MAP area is positioned only at the one partition, control overhead of the partition where the corresponding MAP area is positioned increases.